In a flow cell for use in making fluorescence measurements, a window must be provided of sufficient size to permit entry into the cell of exciting radiation through a large solid angle (called the "acceptance" angle), and to permit observation of the resulting fluorescence emissions along a line at a large angle (typically 90.degree.) from the path of the exciting radiation. Thermostatted flow cells for fluorescence measurements have been used in the past in applications where precise temperature control is important. Thus, in the prior art, instruments were known in which a constant-temperature fluid could be circulated through a jacket surrounding the flow cell. Also, thermoelectric devices using the Peltier effect were known to the prior art as temperature controllers.
With such thermostatted flow cells known to the prior art, however, the large mass of the circulating fluid jackets prevented rapid temperature equilibration. Even with devices using thermoelectric temperature controllers, relatively large masses were required for the components in which the thermoelectric devices were embedded, and consequently rapid temperature equilibration could not be provided.
For typical thermostatted flow cells known to the prior art, the time required for liquid in the flow cell to reach 37.degree. C. from ambient room temperature was on the order of 15 seconds or longer. Furthermore, there were no thermostatted microflow cells (i.e., flow cells having a volume of less than 20 microliters) available for use in making fluorescence measurements.